In vitro performance of two-piece zirconia implant systems for anterior application

Abstract

Objectives

To investigate the influence of the implant–abutment connection on the long-term in vitro performance and fracture resistance of two-piece zirconia implant systems for anterior application.

Methods

Six groups of two-piece zirconia implant systems ( n = 10/group) with screw-retained (5×) or bonded (1×) connections were restored with full-contour zirconia crowns. A two-piece screw-retained titanium system served as reference. For simulating anterior loading the specimens ( n = 8/group) were mounted at an angle of 135° in the chewing simulator, and subjected to thermal cycling (TC: 2 × 9000 × 5°/55 °C) and mechanical loading (ML: 3.6 × 10 6 × 100 N). Failed restorations were examined (scanning electron microscopy). Fracture resistance and maximum bending stress of surviving restorations were determined. 2 specimens per group were loaded to fracture after 24 h water storage without TCML. Data were statistically analyzed (ANOVA; Bonferroni; Kaplan–Meier-Log-Rank; α = 0.05).

Results

The bonded zirconia system and the titanium reference survived TCML without any failures. Screw-retained zirconia systems showed fractures of abutments and/or implants, partly combined with screw fracture/loosening. Failure frequency ( F ) varied between the groups ( F = 8×: 3 groups, F = 3×: 1 group, F = 1×: 1 group). The Log-Rank-test showed significant ( p = 0.000) differences. Fracture forces and maximum bending stresses (mean ± standard deviation) differed significantly (ANOVA: p = 0.000) between 233.4 ± 31.4 N/317.1 ± 42.6 N/mm 2 and 404.3 ± 15.1 N/549.2 ± 20.5 N/mm 2 . Fracture forces after TCML were similar to 24 h fracture forces.

Significance

Screw-retained two-piece zirconia implant systems showed higher failure rates and lower fracture resistance than a screw-retained titanium system, and may be appropriate for clinical anterior requirements with limitations. Failures involved the abutment/implant region around the screw, indicating that the connecting design is crucial for clinical success.

Introduction

The clinical success of two-piece titanium implants has been well-proven over many years . However, the esthetic outcome in anterior regions may be limited by gingival discoloration in patients with thin peri-implant mucosa or soft tissue recession. Optimization of soft tissue esthetics and the current demand for metal-free materials make zirconia an attractive alternative to titanium in implantology . Zirconia is marked by comparable osseointegration as for titanium implants , good stabilization of the soft tissues, low plaque retention , as well as white color and the possibility of staining zirconia in tooth or gingival colors. However, clinical success does not only depend on superior esthetics and a successful osseointegration but also on the performance of the prosthetic suprastructure. As the mechanical properties of zirconia as a brittle material differ from metals , proven geometries of titanium implants and abutments cannot be directly transferred to viable zirconia implant designs without limitations . Although providing high strength and structural reliability, zirconia is vulnerable to bending, low-temperature degradation and subcritical crack growth . So far, the majority of zirconia implant systems presented one-piece systems or bonded two-piece systems . Screw-retained implant–abutment connections as common for titanium implants are wishful but are difficult to realize as thin-peaked zirconia fittings might be exposed to easy fracture. Furthermore, fitting inaccuracies of patient-specific abutments that are attributed to the fabrication process (CAD/CAM-fabrication or sintering) might affect the stability. The challenge of designing a reliable screw-retained connection for zirconia abutments connected to titanium implants has previously been investigated, and the optimal connection type is still controversially discussed .

For metal-free zirconia implant–abutment systems efforts to develop a reliable connection have resulted in the introduction of two-piece zirconia implants with different connecting geometries and screws. Although the different types of zirconia implant–abutment systems are partly already applied clinically, there is only little scientific information . Clinical data are even less and mainly consist of case reports and are restricted to short clinical observation periods . Prior to the routine clinical application of new two-piece zirconia implant designs as an alternative to common titanium systems in the anterior region, in vitro thermal cycling and mechanical loading may allow to predict their mechanical performance and resistance against hydrolytic effects in a simulated clinical situation. A long-term testing might stimulate fatigue failures and give detailed information on possibly appearing errors. In cases without any catastrophic failures, a subsequent static fracture test may help locate initiated weak points and allow a comparison of the individual systems.

This study aimed to investigate the influence of different zirconia implant–abutment combinations on the in vitro performance during a long-term chewing simulation and subsequent fracture testing, and give a first prediction of their clinical applicability in comparison to a well-proven titanium implant system. Therefore, the hypothesis of this investigation was that different two-piece zirconia implant systems show in vitro performance comparable to a common two-piece titanium implant system, and fracture resistance that may be appropriate for clinical anterior requirements.

Materials and methods

Six groups of different two-piece zirconia implant systems were restored with full-contour zirconia crowns ( n = 10 per group). Most of the groups represented experimental implant systems, differing in the connection type (screw-retained, bonded), the manufacturing process (HIP: hot isostatic pressing, biaxial pressing, CIM: ceramic injection molding) or the type of zirconia (TZP: tetragonal zirconia polycrystal, ATZ: alumina-toughened zirconia). A well-proven two-piece titanium system served as a reference. For testing the influence of the implant (1. letter: Z = zirconia, T = titanium) and the abutment (2. letter: Z = zirconia, T = titanium) materials, and the connection types (3. letter: S = screw-retained, B = bonded) seven groups were defined (detailed information: Table 1 ):

  • 1 ZZB: bonded zirconia implant system, ATZ-HIP.

  • 2 ZZS: screw-retained zirconia implant system, ATZ-HIP.

  • 3 ZZS: screw-retained zirconia implant system, TZP.

  • 4 ZZS: screw-retained zirconia implant system, TZP.

  • 5 ZZS: screw-retained zirconia implant system, TZP, biaxial-pressed.

  • 6 ZZS: screw-retained zirconia implant system, TZP-CIM.

  • 7 TTS (reference group): screw-retained titanium implant system.

Table 1
Overview of implant systems, differing in the implant (1. letter: Z = zirconia, T = titanium) and the abutment (2. letter: Z = zirconia, T = titanium) materials, the connection types (3. letter: S = screw-retained, B = bonded), the zirconia types (ATZ: alumina-toughened zirconia, TZP: tetragonal zirconia polycrystal), and the manufacturing processes (HIP: hot isostatic pressing, biaxial pressing, CIM: ceramic injection molding).
System Name/manufacturer Material implant/abutment Connection Implant diameter × length [mm] Tissue/bone level (TL/BL)
1 ZZB Experimental ceramic implant with triangular bonded connection Zirconia/zirconia
(ATZ-HIP)
Bonded 4.1 × 10.0 TL
2 ZZS Experimental ceramic implant with triangular screw-retained connection Zirconia/zirconia
(ATZ-HIP)
Screw-retained 35 Ncm
(carbon-fiber-reinforced polymer screw)
4.1 × 10.0 TL
3 ZZS Experimental ceramic implant,
Moje Keramik-Implantate, G
Zirconia/zirconia
(TZP)
Screw-retained 25 Ncm
(titanium screw)
3.8 × 11.0 BL
4 ZZS Experimental ceramic implant,
Moje Keramik-Implantate, G
Zirconia/zirconia
(TZP)
Screw-retained 25 Ncm
(titanium screw)
4.6 × 11.0 BL
5 ZZS Biaxial-pressed implant
WL-tec/Cera M, G
Zirconia/zirconia
(TZP, biaxial-pressed)
Screw-retained 25 Ncm
(titanium screw)
4.1 × 10.0 TL
6 ZZS CIM-manufactured implant
WL-tec/Cera M, G
Zirconia/zirconia
(TZP-CIM)
Screw-retained 25 Ncm
(titanium screw)
4.1 × 10.0 TL
7 TTS
Reference
Standard Plus
Straumann, G
Titanium/titanium Screw-retained 35 Ncm
(titanium screw)
4.1 × 12.0 TL

Depending on the availability, standard implant diameters between 3.8 mm and 4.6 mm were chosen. The implants were accurately positioned in sample holders at bone or tissue level, respectively. The natural structure of the jawbone was approximated by two different embedding resins. Polyoxymethylene (POM, Young’s modulus: 2.6 GPa) was used to imitate the cancellous bone, and a 1 mm thick layer of 30% fiber-reinforced polyetheretherketone (PEEK, Young’s modulus: 10.0 GPa) was used to imitate the cortical bone. The Young’s modulus was chosen according to literature data, reporting average values of 1–5 GPa for cancellous bone, and 6–20 GPa for cortical bone . In combination with the appropriate implant system components a consistent length between the PEEK “bone” level and the incisal edge of the crowns was enabled for all groups. For two-piece zirconia and titanium implant systems with a screw-retained connection, the respective prefabricated straight abutments were tightened with a torque gauge using titanium or carbon-fiber-reinforced polymer screws according to the manufacturer’s instructions (25–35 Ncm). The screw preload was controlled after ten minutes and the screws were retightened if necessary. For the bonded zirconia system the connecting zirconia surfaces were cleaned with isopropyl alcohol and the corresponding straight abutments were connected to the implants with an experimental resin-based composite cement. 70 full-contour anterior crowns (tooth 21) of identical external shape and a crown length of 13.0 ± 0.1 mm were manufactured of yttria-stabilized zirconia (Cercon HT, DeguDent, Hanau, G) by using the CAD/CAM (computer aided design/computer aided manufacturing) technique (Cercon eye/art/brain/heat plus, DeguDent). The dimensions of the crowns were designed in such a way that the abutments did not require any preparation, which might have weakened the material. Therefore, the inner and cervical geometry as well as the thickness of the crowns were modified for the different groups. The zirconia crowns were glazed with the corresponding glazing material (Cercon glaze, DeguDent). The inner faces of the crowns were sandblasted (50 μm, 2.0 bar) and adhesively fixed onto the implant/abutment systems with a resin-based composite cement (Panavia F 2.0, Kuraray, J) after cleaning the connecting zirconia surfaces with isopropyl alcohol.

For simulating clinically relevant anterior loading situations, 8 specimens per group were mounted into a chewing simulator (EGO, Regensburg, G) at an angle of 135° between the long axis of the implant and the horizontal plane of the sample holder ( Fig. 1 ), representing an average interincisal angle of people with normal occlusion . Steatite balls ( d = 12 mm CeramTec, Plochingen, G) served as antagonists and were placed in maximal contact position on the palatal surface 2 mm from the incisal edge of the zirconia crowns. Long-term thermal cycling (TC: 2 × 9000 × 5 °C/55 °C; 2 min each cycle) and simultaneous mechanical loading (ML: 3.6 × 10 6 ; 100 N; 1.6 Hz; mouth opening: 2 mm) with online failure-control was performed to simulate and control fatigue failures. According to literature data of chewing simulations with zirconia and ceramic restorations, the applied parameters might simulate a maximum of fifteen years of oral service . During the simulation, all implant groups were optically monitored, apparent failures were documented, and failed specimens were excluded from the further simulation process. Scanning electron micrographs (SEM; magnification: 40–200×; working distance: 20.4 mm; voltage: 5–10 keV; low vacuum; Quanta FEG 400, FEI Company, Hillsboro, USA) were made for fractographic failure analysis. Implant systems that survived TCML without any failure were subsequently loaded until fracture with a universal testing machine (Zwick 1446, Ulm, G; v = 1 mm/min). In analogy to chewing simulation, load was applied at an angle of 135° with the loading stamp centrally positioned at the palatal surface 2 mm from the incisal edge. A tin foil (0.25 mm, Dentaurum, Ispringen, G) between crown and loading stamp prevented force peaks. To evaluate the impact of fatigue testing on fracture resistance, 2 specimens per group were stored in distilled water for 24 h and were loaded until failure without preceding TCML. Implant/abutment systems were optically examined after fracture testing, and the failure mode was documented.

Fig. 1
Design of testing apparatus and loading situation.

Fracture force was determined and maximum bending stress σ b was calculated using the formula:

<SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='σb=MW,’>σb=MW,σb=MW,
σ b = M W ,

with M being the bending moment: M = F × l × sin 45°, and W being the moment of resistance: W = ( π /32) × D 3 ( D : diameter of the implant).

Calculations and statistical analysis were done with SPSS Statistics 23 (IBM, Armonk, USA). Power calculation (G*Power 3.1.3, Kiel, G) provided an estimated power of >95% using eight specimens per group. Means and standard deviations of fracture force and bending stress were calculated and analyzed using one-way analysis of variance (ANOVA) and the Bonferroni-test for post-hoc analysis. Survival performance was calculated with the Kaplan–Meier Log-Rank test. The level of significance was set to α = 0.05.

Materials and methods

Six groups of different two-piece zirconia implant systems were restored with full-contour zirconia crowns ( n = 10 per group). Most of the groups represented experimental implant systems, differing in the connection type (screw-retained, bonded), the manufacturing process (HIP: hot isostatic pressing, biaxial pressing, CIM: ceramic injection molding) or the type of zirconia (TZP: tetragonal zirconia polycrystal, ATZ: alumina-toughened zirconia). A well-proven two-piece titanium system served as a reference. For testing the influence of the implant (1. letter: Z = zirconia, T = titanium) and the abutment (2. letter: Z = zirconia, T = titanium) materials, and the connection types (3. letter: S = screw-retained, B = bonded) seven groups were defined (detailed information: Table 1 ):

  • 1 ZZB: bonded zirconia implant system, ATZ-HIP.

  • 2 ZZS: screw-retained zirconia implant system, ATZ-HIP.

  • 3 ZZS: screw-retained zirconia implant system, TZP.

  • 4 ZZS: screw-retained zirconia implant system, TZP.

  • 5 ZZS: screw-retained zirconia implant system, TZP, biaxial-pressed.

  • 6 ZZS: screw-retained zirconia implant system, TZP-CIM.

  • 7 TTS (reference group): screw-retained titanium implant system.

Table 1
Overview of implant systems, differing in the implant (1. letter: Z = zirconia, T = titanium) and the abutment (2. letter: Z = zirconia, T = titanium) materials, the connection types (3. letter: S = screw-retained, B = bonded), the zirconia types (ATZ: alumina-toughened zirconia, TZP: tetragonal zirconia polycrystal), and the manufacturing processes (HIP: hot isostatic pressing, biaxial pressing, CIM: ceramic injection molding).
System Name/manufacturer Material implant/abutment Connection Implant diameter × length [mm] Tissue/bone level (TL/BL)
1 ZZB Experimental ceramic implant with triangular bonded connection Zirconia/zirconia
(ATZ-HIP)
Bonded 4.1 × 10.0 TL
2 ZZS Experimental ceramic implant with triangular screw-retained connection Zirconia/zirconia
(ATZ-HIP)
Screw-retained 35 Ncm
(carbon-fiber-reinforced polymer screw)
4.1 × 10.0 TL
3 ZZS Experimental ceramic implant,
Moje Keramik-Implantate, G
Zirconia/zirconia
(TZP)
Screw-retained 25 Ncm
(titanium screw)
3.8 × 11.0 BL
4 ZZS Experimental ceramic implant,
Moje Keramik-Implantate, G
Zirconia/zirconia
(TZP)
Screw-retained 25 Ncm
(titanium screw)
4.6 × 11.0 BL
5 ZZS Biaxial-pressed implant
WL-tec/Cera M, G
Zirconia/zirconia
(TZP, biaxial-pressed)
Screw-retained 25 Ncm
(titanium screw)
4.1 × 10.0 TL
6 ZZS CIM-manufactured implant
WL-tec/Cera M, G
Zirconia/zirconia
(TZP-CIM)
Screw-retained 25 Ncm
(titanium screw)
4.1 × 10.0 TL
7 TTS
Reference
Standard Plus
Straumann, G
Titanium/titanium Screw-retained 35 Ncm
(titanium screw)
4.1 × 12.0 TL
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Nov 23, 2017 | Posted by in Dental Materials | Comments Off on In vitro performance of two-piece zirconia implant systems for anterior application
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